1. Field of the Invention
The present invention relates generally to methods and systems for controlling the mass flow rate of a fluid, and more particularly to the operation of mass flow controllers and mass flow meters for gases and other compressible fluids.
2. Discussion of the Related Art
Many industrial processes require precise control of various process fluids. For example, in the semiconductor industries, mass flow meters and controllers using them are used to precisely measure and control the amount of a process fluid that is introduced to a process chamber. A wide variety of techniques can be measured to mass flow rate in such devices, including thermal, ultrasonic time-of-flight, coriolis, and pressure-based techniques.
Pressure-based mass flow meters use a defined flow restriction to create a pressure drop responsive to the flow to be measured, and use measurements of temperature, the resulting pressure drop, and (for compressible fluids) absolute pressure, combined with knowledge of both fluid properties and the flow restriction, to calculate mass flow rate.
The term “fluid” is used herein to describe any type of matter in any state capable of flow. The term “gas” is used herein to describe any fluid for which density is substantially dependent on absolute pressure, such as ideal or non-ideal gases, vapors, and supercritical fluids. The term “liquid” is used herein to describe any fluid for which density is not substantially dependent on absolute pressure.